Conventional solid golf balls have primarily two functional components, i.e., the core and the cover. The primary purpose of the core is to be the “spring” of the ball or the principal source of resiliency. The primary purpose of the cover is to protect the core. Solid golf balls include two-piece balls and multi-layer balls. Two-piece solid balls are made with a single-solid core, which is usually a cross-linked polybutadiene rubber encased by a hard cover material. The resiliency of the core can be increased by increasing the cross-link density of the core material. As the resiliency increases, however, the compression may also increase making the balls stiffer. Stiffness is a physical attribute defined by load per unit of deflection. In the golf ball art, stiffness is commonly measured using Atti and Rheile “compression” gauges; however, other methods can be used. Multi-layer solid balls generally include multi-layer core constructions or multi-layer cover constructions, and combinations thereof.
Different covers vary in the types of protection they provide, and different cores have different protection requirements. For example, polybutadiene cores in solid balls can be adversely affected by moisture, and their covers should have good moisture barrier properties and should be applied to the cores soon after their formation. On the other hand, if a wound core is exposed to air, the windings may oxidize rapidly and lose their resiliency. As a result, wound balls require covers that protect them from oxidation.
A correlation has been observed between the stiffness of the cover and the resiliency of the ball. Stiff or hard thermoplastic ionomer covers can function as a hoop-stress layer providing both core protection and improved resilience. However, high hoop-stress layer may cause the ball to have a hard feel and to perform less well in greenside play. Ionomers are commercially available as SURLYN® produced by E.I. DuPont de Nemours & Co. of Wilmington, Del., among others, and are commonly used as golf ball covers.
Other polymers can also be used as cover materials. U.S. Pat. No. 6,132,324 discloses a method of making a golf ball having polyurethane cover. This patent is hereby incorporated by reference in its entirety. Polyurethanes have been long recognized as useful materials for golf ball covers. Polyurethane compositions are the product of a reaction between a curing agent and a polyurethane prepolymer, which is itself a product formed by a reaction between a polyol and a diisocyanate. The curing agents are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Since 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. The first commercially successful polyurethane covered golf balls were the Titleist® Professional®, which was released in 1993, and the Spalding® Executive. Subsequently, the Titleist® Pro-V1® ball was introduced successfully in 2000 with a solid resilient solid polybutadiene core, a hard ionomer casing and a polyurethane cover. The Pro-V1 ball provided both professional and amateur players with long distance off of drivers and better control for greenside play.
A drawback from using polyurethane as the cover layer is that aromatic polyurethane lacks color stability when exposed to ultraviolet light. On the other hand, as discussed above ionomer resins can be too rigid as a cover layer, which can negatively affect greenside play. Compression molded solid polybutadiene had been used in one-piece golf balls, and offered good surface durability. This polybutadiene surface also retained its appearance and durability after long term play. However, one-piece golf balls are unable to achieve the performance characteristics required in high performance balls. Furthermore, the surface finish of compression molded polybutadiene is less aesthetically pleasing as polyurethane or ionomer covers.
For golf ball cores, the most common polymer used is solid polybutadiene, and more specifically, polybutadiene having a high cis-isomer concentration. In one example, U.S. Pat. No. 6,302,808 B1 discloses a ball comprising a core encased by an intermediate layer made from a three-layer composite and a cover. The core and the multi-layer intermediate layer are made from solid polybutadiene and provide improved playing characteristics such as spin and overall driving distance. Liquid polybutadiene has been used as a component in core or cover formulations. U.S. Pat. No. 5,096,201 discloses a golf ball composition, which includes solid rubber and a premix of co-crosslinking agents. The premix contains a liquid polymer, e.g., butadiene rubber, and an unsaturated fatty acid. U.S. Pat. No. 5,215,308 discloses a golf ball formulation that includes a base rubber, a co-crosslinking agent and an organic peroxide. The base rubber includes at least 40% of solid polybutadiene cis-1,4 isomer and a liquid polybutadiene and/or liquid polyisoprene. U.S. published patent application no. 2003/0073517 discloses a cover formulation, which includes an ionomer resin, a urethane material and a rubbery elastomer. The rubbery elastomer includes an elastomer and an optional polar-bearing compound, which can be liquid polybutadiene, among many other polymers.
However, the prior art does not disclose a golf ball having a layer, such as cover or casing layers, made from castable liquid polybutadiene.